You can connect your ATA to the house wiring, but you do so at your own risk. This is not supported by Vonage or Cisco.

That being Said, It does work. The rule you follow is

YOU MUST DISCONNECT THE TELCO LINE COMMING INTO YOUR HOUSE

You must do this before you connect the ATA to your wall jack. If you donot disconnect the outside line from the TELCO, you will burnout the ATA (Or so I have been told). The telco line to your house has voltage on it (When they ring your phone it's about 90 Volts I believe).

Once you have the TELCO line disconnected, You can plug the the ATA into the wall jack and now all other phones in your house will be working on the Vonage line. I have 3 other phones and it works fine. I do not know if there is a maximum.

If you need to re-connect the Telco line back up again. You must un-plug the ATA first.